Lower experiment percentage for optional tryserver.chromium.chromiumos builders.

TBR=tandrii@chromium.org

Change-Id: I1b59d76dd763aceefb593e3a018b472ec7cb1215
Reviewed-on: https://chromium-review.googlesource.com/1062774Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559295}

infra/config/branch/cq.cfg

@@ -72,11 +72,11 @@ verifiers {
       name: "master.tryserver.chromium.chromiumos"
       builders {
         name: "linux-chromeos-compile-dbg"
-        experiment_percentage: 5
+        experiment_percentage: 1
       }
       builders {
         name: "linux-chromeos-dbg"
-        experiment_percentage: 5
+        experiment_percentage: 1
       }
     }
     buckets {